1. Field of the Invention
The present invention relates to a device for non-invasive determination of a glucose concentration in the blood of a subject with improved accuracy.
2. Disclosure of the Related Art
In the past, a near-infrared spectroscopic analysis has been known as a method comprising the steps of projecting near-infrared radiation having wavelengths of 800 nm to 2500 nm to a target, receiving a resulting radiation, i.e., a transmission light or reflection light from the target, and performing a spectrum analysis of the resulting radiation. This method has the following advantages:
(1) Since a low energy electromagnetic wave is used, it is possible to avoid the occurrence of radiation damage of the target. PA0 (2) An absorption by water of near-infrared radiation is smaller than that of infrared radiation, therefore, it is possible to select an aqueous solution as the target. PA0 (3) It is possible to perform the spectroscopic analysis under various physical states of the target, i.e., a solid state such as powder or fiber, a liquid state and a gas state.
As an application of the near-infrared spectroscopic analysis, it is increasingly utilized for non-invasive determination of a glucose concentration in the body of a subject. For example, U.S. Pat. No. 5,379,764 discloses a method of determining a glucose concentration in the blood of a patient. FIG. 39 is a schematic diagram of a device for practicing the method. Near-infrared radiation is provided from a light source 1Y which is capable of producing the radiation over the range of 700 nm to 3000 nm, and projected on a skin 9Y of the patient through a first lens system 2Y and an input radiation conductor 10Y, e.g., optical fibers. A resulting back-scattered radiation emitted from the inside of the skin 9Y is received by a sensing radiation conductor 20Y. The received radiation is provided through a second lens system 3Y to a spectrum analyzer/detector 30Y, and then a data processor 40Y to make a spectrum analysis of the received radiation and determine the glucose concentration in the blood of the patient by using a multivariate analysis. The obtained glucose concentration is sent to a display monitor 50Y and an output recorder 60Y.
On the other hand, PCT Publication No. WO94/10901 discloses a method for determining a glucose concentration in a biological matrix. As shown in FIG. 40, the method uses a light emitter (not shown) for projecting a primary light 1Z of near-infrared radiation at a projection site 10Z to a skin 9Z of the biological matrix, and first and second light receivers (not shown) for sensing resulting secondary lights (2Z, 3Z) emitted from the skin 9Z at first and second detection sites (20Z, 30Z). The first detection site 20Z is spaced from the projection site 10Z by a distance D1 of at least 0.5 mm, and preferably 1 mm. The second detection site 30Z is spaced from the projection site 10Z by a distance D2 (#D1) of 30 mm or less. A light path passing through the skin 9Z between the projection site 10Z and the first detection site 20Z is different from the light path passing through the skin between the projection site and the second detection site 30Z. Therefore, the glucose concentration can be derived from the dependence of the intensity of the secondary light from the relative position of the projection site and the detection site.
By the way, each of these measurements of the glucose concentration is performed by projecting near-infrared radiation on a skin of a forearm or finger of the subject. The skin is generally composed of three layers, that is, an epidermis layer having a thickness of about 100 .mu.m, a dermis layer having a thickness of about 1 mm and positioned under the epidermis layer, and a subcutaneous-tissue layer including adipose cells and positioned under the dermis layer. These layers shows different glucose concentrations. Therefore, if variations in glucose concentration on such a complex skin structure is carefully analyzed, it will be possible to determine the glucose concentration in the blood with improved accuracy. Thus, there is room for further improvement on the measurements of the glucose concentration of the prior art.